Change default of $variant param of idn_to_ascii() and idn_to_utf8()

Change default of $variant param of idn_to_ascii() and idn_to_utf8()

According to the “Deprecate and remove INTL_IDNA_VARIANT_2003” RFC[1],
we change the default of the $variant parameter of `idn_to_ascii()` and
`idn_to_utf8()` from `INTL_IDNA_VARIANT_2003` to
`INTL_IDNA_VARIANT_UTS46`.

[1] <https://wiki.php.net/rfc/deprecate-and-remove-intl_idna_variant_2003>